14

2016

Investigation into Introduction of New 

World Screwworm into Florida Keys
1

Table of Contents

Background ...................................................................................................................................................2

Objective:..................................................................................................................................................2

Descriptive epidemiology of the outbreak ...................................................................................................3

Key deer....................................................................................................................................................3

Companion animals ..................................................................................................................................7

Retrospective case finding ....................................................................................................................7

Investigation into known pathways for introduction of New World Screwworm........................................9

Undocumented human and animal movements....................................................................................10

Legal human and animal movements.....................................................................................................11

Discussion....................................................................................................................................................13

Citations......................................................................................................................................................16
2

Background

On September 30, 2016, entomologists 

at the United States Department of Agriculture, 

Animal and Plant Health Inspection Service, 

Veterinary Services (USDA-APHIS-VS) National 

Veterinary Services Laboratories (NVSL) in Ames, 

Iowa, confirmed a diagnosis of screwworm 

myiasis in a Key deer (Odocoileus virginianus 

clavium) from Big Pine Key, Florida and identified New World Screwworm (Cochliomyia hominivorax; 

NWS) as the causative agent. An investigation was initiated by USDA-APHIS-VS to determine the origin 

of the screwworm infestation and try to understand pathways through which introduction may have 

occurred. On November 1, 2016, three epidemiologists from the Center for Epidemiology and Animal 

Health (CEAH) arrived in Florida to join the incident management team in assisting the Florida 

Department of Agriculture and Consumer Services (FDACS) in the investigation and response. The 

investigation focused on descriptive epidemiology of the initial outbreak in deer and pets, retrospective

case finding, and interviews with individuals whose work occurs in relation to known pathways for 

screwworm introduction.

Objective: To improve our understanding of the time course of screwworm introduction into the Florida 

Keys, to identify retrospective cases that may have not been recognized previously, and to examine risk 

pathways for screwworm fly introduction and possible changes in risk. 
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Descriptive epidemiology of the outbreak

Key deer

The first known observation of an infested Key deer occurred on July 5, 2016, on Big Pine Key. The 

deer was examined by a wildlife biologist from the National Key Deer Refuge, and the presence of maggots 

in a leg wound was noted. Since Key deer are an endangered subspecies, all mortality events are recorded 

and geo-located by the staff of the National Key Deer Refuge. 

Beginning in August, the refuge staff noticed an unusual increase in buck mortalities that were 

not road-related. By September, the mortality rate reached levels that were approximately twice as high 

as normal (Figure 2). That month, refuge personnel visited with a local veterinarian regarding the cases 

and learned that the veterinarian had previously seen a few cases of fly strike, or myiasis, in pets. The

refuge staff decided to submit maggot samples for identification. New World screwworm was confirmed 

by entomologists at the NVSL on September 30th, 2016. 

The estimated numbers of infested deer by week are shown in Figure 1, Panel A. The locations of 

the first cases of Key deer mortalities from screwworm myiasis for the month of July are shown with red 

dots in Figure 1, Panel B. The carcasses of these deer were transported to a site in the northern part of 

Big Pine Key to be returned to the landscape, consistent with the refuge’s normal procedures for deer 

mortalities. The location of the disposal site is shown as a black star in Figure 1, Panel B. The first cases 

were reported from Big Pine Key in July, with cases reported from No Name key in August. By October,

NWS infestation among Key deer was reported on several Keys. Following the official diagnosis of 

screwworm myiasis, infested carcasses were no longer returned to the landscape and alternative disposal 

methods were used.

In 2016, deer mortalities ranged from 7 to 30 deaths per month through the end of September

(Figure 2). Overall, a gradual increase in monthly deer mortalities was seen throughout the year, with a 

small spike in mortalities in May, and the majority of those mortalities were attributed to road kills. 
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Because all of these deer were inspected and recorded as explained mortalities, it is unlikely that 

screwworm larvae were present and not noted. A large increase in mortality numbers began in 

September and peaked in October. October had the highest mortalities of Key deer by any cause (98 total 

deaths) recorded in any month over the past 10 years (Figure 2). A comparison of monthly mortality data 

from 2016 and median monthly mortality data from 2010-2015 suggests that the early months of 2016 

experienced normal Key deer mortality prior to the outbreak (Figure 2). In addition, interviews with 

wildlife biologists at the refuge suggested that the fawn crop in the spring of 2016 was excellent. If 

screwworm had been present in large numbers during the fawning season, the biologists felt they would 

have seen myiasis cases associated with the naval area of newborns. No cases of myiasis in newborn Key

deer fawns were observed in 2016.
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Figure 1. The number of deer infested by week (Panel A) in the Lower Florida Keys, and the spatial 

distribution of infested deer (Panel B) and pets (Panel C) by month in 2016. The red dots are infested 

deer, the brown dots are infested dogs, pink dots are infested pigs, and the gray dots are infested cats.

A.

B.

C.
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Figure 2: Key deer mortality by month, 2006-2015; median number of deaths by month versus 2016deaths 

by month. 

0

20

40

60

80

100

120

Number of deaths

Month

Key Deer Mortality by Month

median (2006-2015) 2016
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Companion animals

Several cases consistent with screwworm infestation were identified by local veterinarians prior to this 

investigation. The spatial progression of these cases is shown in Figure 1, Panel C. However, additional 

cases might not have been recognized by the local veterinary community. In order to identify these 

possible, additional historic cases, a questionnaire was developed and administered to veterinarians and 

animal facilities throughout the Keys. 

Retrospective case finding

The questionnaire for veterinarians and animal facilities inquired about any unusual cases of 

maggot infestation that might have been seen during 2016. For any mentioned suspect cases, the owners 

of the animals were further contacted in order to obtain additional history, demographic, and clinical 

details. Throughout the Florida Keys, 22 animal facilities were visited. This survey included 22 facilities: 

17 (77%) veterinary clinics, 4 (18%) pet animal shelters, and 1 (5%) petting zoo. The veterinary clinics see 

a reported median of 60 (range: 5 to 300) animals per week routinely. All of the pet animal shelters and 

the petting zoo completed the questionnaire, while 15/17 (88%) of veterinary clinics completed the 

questionnaire. The two veterinary clinics not completing the questionnaire were both mobile veterinary 

services, and both had disconnected or non-working phone numbers. 

A majority of the respondents to the veterinary clinic questionnaire, 17/20 (85%), reported that 

they had not seen any cases of myiasis or maggot-infested wounds, while 3/20 (15%) reported they had 

seen one or more cases of myiasis or maggot-infested wounds throughout 2016. None of the animal 

shelters or animal control facilities reported seeing any myiasis cases over the course of 2016. All of the 

animal shelters have relationships with local veterinary practitioners, and any animal arriving at a shelter 

needing clinical care would have been taken to a local veterinarian. A local organization that works with 

the feral cat population also did not report seeing any cases of myiasis or open wounds with maggots in 
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2016. Further, they also have a relationship with a local veterinarian, and any animal requiring veterinary 

care would have been referred to the local veterinarian. 

 Three (15%) of the veterinary clinics completing the questionnaire reported that they had seen 

at least one case of myiasis in 2016. The nine cases reported from the veterinary clinics included seven 

dogs, one cat, and one pig (Table 1). One of the clinics reporting cases had seen only one case in a canine, 

one clinic reported seeing three canines, and another clinic reported seeing three canines, one porcine, 

and one feline. The reported myiasis cases occurred between July 22, 2016, and November 5, 2016. All 

of the reported myiasis cases occurred after July 5, 2016, which is when the first suspected case in a 

Florida Key deer was seen.

Table 1: Species, date examined by the veterinarian, and route of case identification for suspect and 

confirmed cases in companion animals. Three animals (3/9, 33%) were identified by the questionnaire 

and not routine reporting. 

SPECIES DATE EXAMINED ROUTE OF CASE IDENTIFICATION

DOG 7/22/16 Vet report

DOG 7/25/16 Questionnaire

PIG 7/30/16 Vet report

DOG 8/6/16 Questionnaire

DOG 8/26/16 Vet report

DOG 8/28/16 Questionnaire

DOG 9/15/16 Vet report

CAT 10/17/16 Vet report

DOG 11/5/16 Vet report

The animal owners of the nine cases reported from veterinary clinics were contacted for follow 

up by Veterinary Medical Officers with the USDA-APHIS-VS to obtain additional information. Three 

attempts were made to contact the owners, after which time, if there was no response they were 

classified as a non-responder. Non-responders accounted for 4/9 (44%) of owners, while owner surveys 

were completed for 5/9 (56%) of reported cases. 
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The completed owner surveys included four canines and one feline. Among the five respondents, 

3 (60%) were confirmed NWS positive, 1 (20%) was presumptive positive, and 1 (20%) was a case of 

reported maggot infestation with an unknown type larvae. The animals were seen by a veterinarian 

during the period July 22 through November 5, 2016, and all of the animals had recovered from their 

infestations at the time of interview. None of the five reported using any fly control, heartworm 

prevention, or flea and tick prevention prior to the diagnosis, and only one out of the five had been 

dewormed in 2016. When asked to rank the level of fly problems at the time of illness on a scale of 1 -5 

with 5 being the worst, 4/5 (80%) respondents reported a level 1, while only one respondent reported a 

level 3. In estimating the percentage of time the pet spent outdoors, one respondent reported 1-25%, 

one respondent reported 26-50%, one respondent reported 76-99%, and two respondents reported the 

pet spent 100% of its time outdoors. Most of the respondents (4/5, 80%) reported that they live near 

water. None of the animals had traveled outside the Florida Keys in 2016, and none had contact with any 

animals known to have traveled outside the Florida Keys. 

Investigation into known pathways for introduction of New World 

screwworm

There are several known pathways for introduction of NWS, most of which rely on larvae being 

carried into the United States on a human or an animal. Humans may have entered the United States 

unknowingly infested with NWS larvae, or they may have unknowingly brought in animals infested with 

NWS larvae. These movements could have occurred either legally or illegally. To investigate these 

pathways, the appropriate agencies were visited to discuss both human and animal international 

movements directly into the Florida Keys. 

Other potential pathways such as the movement of flies or larvae on products, such as hides or 

skins, or within conveyances, such as boats, were not explored as part of this investigation. Natural 

dispersal, including long-distance flights across open water, has occurred in the past and was responsible 
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for the rapid annual colonization of United States by the fly from over-wintering areas in southern Texas 

and Mexico prior to eradication (Spradley, 1994). The flight range of individual adult screwworm flies is

between 3 to 300 km depending on environmental conditions and host density, though it is typically 10

to 20 km in warm humid environments (OIE, 2013). Wind and other climatic conditions, habitat factors,

and the availability of resources, such as food and hosts, all influence fly dispersal, making clear 

associations between fly incursions and weather-related events difficult (Mayer and Atzeni, 1993). This 

route has not yet been explored as part of this investigation due to the uncertainty around introduction 

times or locations.

Undocumented human and animal movements

The local Customs and Border Protection (CBP) office in Marathon was visited, but unfortunately,

specific data regarding the numbers and locations of illegal boat landings, and possible interdictions that

included an animal, were not readily available. Whenever there is an interdiction involving an animal, CBP 

reports the animal to the Centers for Disease Control and Prevention (CDC) quarantine station in Miami, 

which then collects owner and location information and contacts the local animal control agency, which

then picks up the animal. The animal is taken to a local shelter, and if necessary, to a local veterinarian. 

The animal remains in quarantine at the shelter or veterinary clinic until it can be returned to its owner or 

adopted out. Of the animal shelters interviewed, only one reported receiving any animals (2-3 animals to 

date) in 2016 from CBP interdictions, and none of these animals had wounds requiring veterinary care.

Although the Coast Guard’s website on migrant interdictions appears to be unavailable, several 

independent local news sources have cited a dramatic increase in boat landings and interdictions during 

the summer of 2016. During the week of July 19th, for example, the Miami Herald reported 107 Cuban 

immigrants arrived in the Florida Keys in the span of a single week (Herrera, 2016). They estimated that 

successful boat migrations were 106% greater than the same time last year. The number of Cuban 

migrants in particular has surged since late 2014, when the U.S. announced that it was reestablishing ties 
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with Cuba. The Pew Research Center found that 46,635 Cubans have entered the U.S. via ports of entry 

in the first 10 months of 2016, already exceeding the total number of migrants from 2015 and almost 

double the number of migrants from 2014 (Krogstad, 2016). While Cubans account for the majority of 

interdictions, migrants from Haiti and the Dominican Republic are the second largest group (United States 

Coast Guard, 2016). All three of these countries are known to be infested with screwworms, and the 

undocumented movement of people and pets from these countries continues to pose a risk to the U.S for 

introduction of NWS. 

Legal human and animal movements 

The legal entry of people and pets through airports and water ports represents another potential 

pathway of introduction of NWS. The CBP agricultural specialist at the Key West Airport was contacted in 

order to better understand the volume and management of animals coming through Key West Airport. 

The only international flights received at Key West Airport are private flights. Between January and July 

of 2016, three dogs were cleared for entry to the US from international flights. Two of the dogs came 

from Nassau, Bahamas, and one dog came from Grand Cayman Islands. These countries do not currently 

report the presence of NWS, and all three dogs were examined and reported to be in good physical 

condition at the time of entry. The Marathon municipal airport also receives only private aircraft; 

however, the reporting and inspection of animals there is variable. Following the detection of 

screwworms on the Florida Keys, a new procedure was developed, which requested that the NWS Incident 

Command Post (ICP) be notified when an animal is received at the Marathon municipal airport from an 

international flight. Historic information on incoming pets during the first six months of 2016 was not 

available. The inspection and reporting of animals arriving on international private watercraft also appears 

to be variable, and records of pets entering on private watercraft were not available.
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In order to assess the possibility of human introduction, representatives from the Monroe County 

Public Health Department were contacted. They reported no human cases of NWS in 2016 or cases of 

wounds with severe myiasis that could be consistent with NWS. 

Since persons may or may not seek medical attention for themselves or their pets, a public 

outreach survey was launched to allow the public to report any unusual myiasis cases in people or pets 

that could have been screwworms during 2016. The survey link was distributed throughout the Florida 

Keys on flyers and via websites. The survey is currently active, and people can continue to submit reports. 

Discussions with field staff involved in the outbreak response identified one local person who felt that 

s/he may have seen a screwworm-infested deer as early as March; however, no additional information 

was available. To date, five responses have been received for the public survey. All five involved reports 

of deer with clinical signs consistent with NWS (open woundsin live animals, presence of maggots, strange 

behavior including stumbling, head shaking, and depression). The earliest report references an animal 

seen in late June or early July on Big Pine Key. Other reported sightings of possibly infested deer occurred 

on July 15th, August 8th, sometime in August, and September 29th. 
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Discussion

Before the 2016 Florida Keys outbreak, NWS 

infestations capable of sustained reproduction had been 

eradicated from the United States, though periodic incursions 

have occurred. The first known Florida Key deer likely to be 

infested with NWS larvae was observed on July 5, 2016. The first 

presumptive positive canine infestation was believed to have 

occurred on July 22, 2016. There were no reported cases in 

companion animals that raised suspicion of NWS from 

veterinary clinics in the Florida Keys prior to July 22, 2016. CBP 

did not report any interdictions among aircraft or watercraft 

with suspected diseased animals, and the local public health department of Monroe County did not report 

any human cases of NWS infestation among humans. 

Given the high infestation rate among Key deer, and the lack of reported infested companion 

animals observed prior to the first Key deer infestation, NWS infestation among companion animals was 

probably a spillover event from infested Key deer. In addition, none of the owners of companion animals 

identified as infested reported travel of a pet outside of the Florida Keys during 2016, making the case for 

the spillover from deer more likely. The descriptive epidemiology of the outbreak in deer would suggest 

that only very low levels of infestation could have been present on Big Pine Key prior to July, and additional 

information on the 2016 fawn crop and overall deer population has been requested. 

The population doubling time for the screwworm fly can range from 54 to 139 days in densityindependent growth conditions, such as those likely to prevail during outbreaks in initially screwwormfree territory. Under ideal conditions, it is plausible that screwworm outbreak populations could double 

in 1-2 months. However, the time needed can vary based a wide variety of factors, such as temperature, 
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sunlight, moisture, etc. The typical life cycle of New World screwworm is shown in Figure 3 (Fernandez 

and White, 2010). 

An adult female fly lays eggs in an open wound on a host. After 8-12 hours, larvae emerge, enter 

the wound, and begin to feed. After 5-7 days, the mature larvae exit the wound and burrow into 

surrounding soil, where they pupate for 7-60 days, depending on temperature and environmental 

conditions. After pupating, adult flies emerge, feed, and rest for approximately 3-5 days, at which point 

they are ready to mate and seek out a host for egg laying. In tropical conditions (84° F) the entire cycle 

may be as short as 18 days (Schlater and Mertins, 2008).

Figure 3. Life cycle of New World Screwworm from Fernandez and White, 2010.1

 

1 Photo source: USDA APHIS. STOP Screwworms: Selections from the Screwworm Eradication Collection, Special 

Collections, National Agricultural Library.http://specialcollections.nal.usda.gov/screwworm/index.
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Given the difficulty of detecting sparse outbreak populations and the presence of undetectable 

phases in the life cycle of the fly, introduced screwworm populations could easily double several times 

before discovery (Matlock and Skoda, 2009). Observations in the field of the stages of larvae found in the 

wounds of deer in early October were consistent with NWS being present on Big Pine Key since June; 

however NWS could have been present on other Keys at low levels for several months before then. 

There were limited data available regarding animals arriving into the Keys legally from 

international destinations; however, the available information suggests that this is an infrequently 

identified occurrence and remains a low likelihood of disease introduction. The large increase in 

movements of undocumented people from screwworm-infested countries during mid-2016 remains a 

concern. The genotyping of the fly could provide additional insight into the most likely source and route 

of introduction, and the information described in this report should be re-evaluated as more information 

becomes available from that analysis. 

The pathway of introduction for this occurrence of NWS has not been identified, though several 

plausible pathways exist and could not be ruled out. The information gathered suggests that NWS was 

not present prior to 2016 and was most likely introduced into the Keys in the spring of 2016, where and 

when the presence of a very suitable host (Key deer) allowed the fly to become established.
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Citations

Fernández PJ, White WR. 2010. Atlas of Transboundary Animal Diseases. Paris: World Organization

for Animal Health. 218-219.

Herrera, C. 107 Cuban Migrants Have Arrived in the Florida Keys in the Last Week. Miami Herald. N.p., 

19 July 2016. Web. 05 Dec. 2016. Available at: 

http://www.miamiherald.com/news/local/community/florida-keys/article90555502.html

Krogstad JM. Surge in Cuban Immigration to U.S. Continues into 2016. Pew Research Center, 05 Aug. 

2016. Web. 05 Dec. 2016. Available at: http://www.pewresearch.org/fact-tank/2016/08/05/cubanimmigration-to-u-s-surges-as-relations-warm/

Matlock RB, Skoda, SR. 2009. Mark – recapture estimates of recruitment, survivorship and population 

growth rate for the screwworm fly, Cochliomyia hominivorax. Medical and Environmental Entomology 

23 (Suppl 1), 111-125.

Mayer DG, Atzeni MG. 1993. Estimation of dispersal distances for Cochliomyia hominivorax (Diptera: 

Calliphoridae). Environmental Entomology 22, 368–374.

OIE. 2013. Screwworm (Old World and New World). Retrieved from 

http://www.oie.int/fileadmin/Home/eng/Animal_Health_in_the_World/docs/pdf/Disease_cards/SCRE

WWORM.pdf

Schlater J, Mertins J. 2008. Screwworm Myiasis. In Foreign Animal Diseases (383-390). St Joseph, MO: 

USAHA.

Spradbery JP. 1994. Screw-worm fly: a tale of two species. Agricultural Zoology Reviews 6, 1–62.

United States Coast Guard. Alien Migrant Interdiction. Department of Homeland Security, 12 Jan. 2016. 

Web. 5 Dec. 2016. Available at: https://www.uscg.mil/hq/cg5/cg531/AMIO/amio.asp
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Authors: Amy Delgado, Morgan Hennessey, David Hsi

For more information about this report, please contact:

Amy Delgado

Director, Monitoring and Modeling Unit

Center for Epidemiology and Animal Health

2150 Centre Ave, Building B

Fort Collins, CO 80526

Phone (970) 494-7302